1. Field of the Invention
The present invention relates to evaluation of signals read from an optical recording medium, and in particular, to evaluation of signals that are read using a PRML detection method.
2. Description of the Related Art
Conventionally, optical recording media such as CD-DAs, CD-ROMs, CD-Rs, CD-RWs, DVD-ROMs, DVD-Rs, DVD+/−RWs, DVD-RAMS, and the like are widely used to watch digital moving image contents and record digital data. Conversely, the recording capacity required of these kinds of optical recording media grows with each passing year and the so-called next-generation optical discs, which can store massive amounts of both moving images and data, have come into commercial use to meet such a requirement. In the next-generation optical discs, the wavelength of a laser beam used for recording and reading is shortened to 405 nm in order to increase their recording capacity.
In the Blu-ray Disc (BD) standard, being one of the next-generation DVD standards, for example, the numerical aperture of an objective lens is set to 0.85 in order to record and read 25 GB of data on and from a single recording layer.
However, the size of moving image and computer data is expected to further increase in the future. Therefore, it is contemplated to increase the capacity of a disc by reducing the size of recording marks so that the linear recording density of each layer is increased.
Meanwhile, under optical recording-reading conditions in which a laser beam having a wavelength of λ and an objective lens having a numerical aperture of NA are used, a so-called resolution limit exists. Specifically, when an encoded signal contains a sequence of a recording mark and a space each having a size equal to or less than 1.0×λ/4/NA, the amplitude of the read signal from the train of recording marks and spaces becomes substantially zero. In the current CD, DVD, and BD standards, the size of the minimum recording mark in an encoded signal is greater than 1.0×λ/4/NA, and therefore the resolution limit has not been reached. Hence, an amplitude sufficient for reading a signal can be obtained for any combination of recording a train of recording marks/spaces using an appropriate equalizer. Therefore, by slicing a read signal at a certain voltage level, the quality of the read signal can be evaluated according to the positional information (edge jitter) of the intersection of the slice level and the amplitude curve of the read signal.
According to a study undertaken by the inventors, although the study was publicly unknown at the time of filing of the present application, when the size of recording marks is reduced such that the minimum size thereof is equal to or less than 1.1×λ/4/NA, the amplitude of a signal from a sequence of a recording mark and a space each having the minimum size is below a practically acceptable level. As described above, when the size of the recording marks is reduced to 1.0×λ/4/NA or less, the amplitude becomes essentially zero due to the resolution limit. In this case, the inventors have also found that signal evaluation using the edge jitter cannot be carried out.
A technique for avoiding such a problem is known as a PRML (Partial Response Maximum Likelihood) detection method and in this method a PR equalizer and an ML decoder (such as a Viterbi decoder) are used. The PR equalizer has a function of correcting a real read signal to match the corrected signal to a reference PR characteristic. One coefficient used for this correction is called an equalization coefficient, and a plurality of equalization coefficients corresponding to different amplitudes of the read signal are provided in the PR equalizer.
In the PRML detection method, when a PR (1, 2, 1) characteristic with a constraint length of 3, for example, is used, an impulse response from a real recorded bit is represented by a sequence having an amplitude of PR (h1, h2, h3). Therefore, in the PR equalizer, the equalization coefficients are used to match the read signal having an amplitude of PR (h1, h2, h3) to the reference PR (1, 2, 1) characteristic, and as such, noise components are eliminated.
The ML decoder computes the deviation of the signal sequence equalized by the PR equalizer from each of all possible ideal responses and selects one ideal response having a minimum cumulative deviation (this ideal response is referred to as a maximum likelihood ideal response). An detection signal is obtained from the maximum likelihood ideal response. In this manner, a correct detection signal can be extracted even when the read signal has a small amplitude and is embedded in noise.
The characteristics of individual optical recording media differ from each other, and the characteristics of optical heads of individual reading devices also differ from each other. In order to cope with these differences, an optimal PR equalizer must be selected, or the equalization coefficients used in the PR equalizer must be adjusted. In addition, an optimal decoder is selected as the ML decoder. In particular, in the future, when the linear recording density per layer is increased, the type of PR equalizer and ML decoder must be very carefully selected using strict criteria.
The determination as to whether or not the PRML detection method is appropriate is often made based on the results of reading experiments using various types of PRML detection methods. However, there is no specific criteria and determination method for optimizing the PRML detection method, so great efforts are required for optimization.